Focused ion beam
Source ions are then generally accelerated to an energy of 1–50 kiloelectronvolts (0.16–8.01 fJ), and focused onto the sample by electrostatic lenses.
LMIS produce high current density ion beams with very small energy spread.
[1] Focused ion beam (FIB) systems have been produced commercially for approximately twenty years, primarily for large semiconductor manufacturers.
As the primary beam rasters on the sample surface, the signal from the sputtered ions or secondary electrons is collected to form an image.
At low primary beam currents, very little material is sputtered and modern FIB systems can easily achieve 5 nm imaging resolution (imaging resolution with Ga ions is limited to ~5 nm by sputtering[2][3] and detector efficiency).
At higher primary currents, a great deal of material can be removed by sputtering, allowing precision milling of the specimen down to a sub micrometer or even a nano scale.
If the sample is non-conductive, a low energy electron flood gun can be used to provide charge neutralization.
FIB-assisted chemical vapor deposition occurs when a gas, such as tungsten hexacarbonyl (W(CO)6) is introduced to the vacuum chamber and allowed to chemisorb onto the sample.
This is useful, as the deposited metal can be used as a sacrificial layer, to protect the underlying sample from the destructive sputtering of the beam.
However, the nanometer-scale resolution of the FIB allows the exact region of interest to be chosen, such as perhaps a grain boundary or defect in a material.
[11] The drawbacks to FIB sample preparation are the above-mentioned surface damage and implantation, which produce noticeable effects when using techniques such as high-resolution "lattice imaging" TEM or electron energy loss spectroscopy.
For a minimal introduction of stress and bending to transmission electron microscopy (TEM) samples (lamellae, thin films, and other mechanically and beam sensitive samples), when transferring inside a focused ion beam (FIB), flexible metallic nanowires can be attached to a typically rigid micromanipulator.
The beam current is generally reduced the smaller the inner circle becomes to avoid damaging or destroying the sample.
[15] The focused ion beam has become a powerful tool for site-specific 3D imaging of sub-micron features in a sample.
As helium ions can be focused into a small probe size and provide a much smaller sample interaction than high energy (>1 kV) electrons in the SEM, the He ion microscope can generate equal or higher resolution images with good material contrast and a higher depth of focus.
In order to get an alternative solution to Ga LMI sources, mass-filtered columns have been developed, based on a Wien filter technology.
The principle of a Wien filter is based on the equilibrium of the opposite forces induced by perpendicular electrostatic and a magnetic fields acting on accelerated particles.
[23] Besides allowing the use of sources others than gallium, these columns can switch from different species simply by adjusting the properties of the Wien filter.
Exploiting FIB from such an unconventional perspective is especially favourable today when the future of so many novel technologies depends on the ability to rapidly fabricate prototype nanoscale magnetic devices.
